Infrared imaging is the process of detecting infrared radiation and producing an image capable of being visualized by the human eye. A typical infrared imaging system includes a number of lenses, a chopper, an infrared detector, signal processing electronics, and a display.
One problem found with prior art infrared imaging systems relates to the intensity of the infrared radiation. This problem is particularly evident in wide-angle infrared imaging systems used in extreme environments. For example, in fire-fighting applications, the intensity of infrared radiation from very hot or burning objects is sometimes great enough that it saturates the infrared detector. It is, therefore, desirable to reduce the intensity of the infrared radiation when needed without adversely effecting image quality.
Prior wide-angle infrared imaging systems have used two to three Germanium (Ge) lenses and have used an external entrance pupil or aperture stop. One example of such a prior wide-angle system having an external entrance pupil is illustrated in U.S. Pat. No. 5,479,292 issued to Yoshikawa et al. In particular, such prior design have typically used an external entrance pupil or aperture stop with a negatively powered Ge lens follow by one or two positive powered Ge lenses. The use of external entrance pupils, however, is often undesirable, for example, in extreme conditions such as in fire-fighting applications. If the entrance pupil is external, the pupil cannot be sealed off without a significant increase in the size and cost of the infrared imaging system. Other prior designs that do not use external entrance pupils have made use of three or more optical elements. However, such designs typically have had poorly defined entrance pupils that are not adequate for the full processing range desired.